Climatological Features of the Standard Ice Thickness at Nanyue Mountain Observatory

Using the icing data measured at Nanyue Mountain Observatory from 1957 to 2012, the temporal variations, abrupt changes, and period variations of standard ice thicknesses and icing days are analyzed. The standard ice thicknesses of different recurrence periods are also calculated. The results showed that the interdecadal variability of standard ice thicknesses in east-west (EW) and north-south (NS) directions were similar and presented a pattern of large-small-large-small-large. Icing days before the late 1960s and after the late 1990s are less than annual mean icing days and icing days larger than the annual mean icing days were found between the late 1960s and the late 1990s. Icing usually happened from October to April of the next year and the icing days in January accounted for the largest portion of icing days in the whole year with the percentage of 31.8%. The abrupt changes of standard ice thicknesses in NS and EW directions were found in the late 1960s and no abrupt change of icing days was found in recent 56 years. Morlet wavelet and Morlet wavelet power spectrum analyses showed that standard ice thicknesses in EW and NS directions both had significant periods of quasi 2-3 years and 4-6 years and the icing days had significant periods of quasi 2-4 years and 5-7 years. Using Pearson III type extreme value distributions, the calculated standard ice thicknesses for the 10-year recurrence period in NS and EW directions were 60.86 mm and 69.57 mm, respectively. For 20-year recurrence period, calculated standard ice thicknesses increased to 69.57 mm (EW) and 72.33 mm (NS). The authors analyzed meteorological differences between the icing days and non icing days from 1 December 2012 to 4 March 2013. The mean air temperature of non icing days had a reduction of 7.8 ℃ in comparison to icing days. The mean relative humidity of icing days was larger than that of non-icing days. Both the mean visibility and wind speed of icing days were smaller than those of non-icing days. The dominant wind direction was north when icing happened, and the south wind usually happened in the non icing days. The average wind speed of icing days was less than that of non-icing days.